Patrologia Latina Database--an
electronic version of the first edition of Jacques-Paul Migne's
Patrologia Latina, published between 1844 and 1855, comprising the
works of the Church Fathers from Tertullian in 200 AD to the death of
Pope Innocent III in 1216.

Oxford English Dictionary--The
most extensive dictionary of the English language giving words, their
meanings and etymologies, from the Middle Ages to the present. The
usage of 231,100 words is illustrated by 2.5 million examples. Updated
with new words quarterly.